I have mixing small batches also.
Also I have been adding a product called 'Penetrol'. I am not sure if is available in the US under that name. Basically it helps paint spread and stick.
I mix 100ml batches using the following formula :
70ml paint
15ml Penetrol
15ml mineral turps.
Initially I wasn't using a primer, but I am using Interlux's own primer called 'Prekote'. What I did notice with this is that is really soaks up the first coat or two so you dont get a gloss finish until about the 3rd coat.
But I am thinking using a primer will give a better result in the end.
Cheers,
Frank.